Horse owners may choose stallions based on their conformation, but it doesn’t seem high on the list for mares when it comes to breeding time, study findings suggest.
The stallion preferences of mares differed considerably depending on whether they were in season or not, the European researchers found.
Out-of-season mares leaned toward older and larger males, but only if visual cues were available. However, once they were in season, their preferences changed, basing their choices mainly on non-visual traits that could not be predicted by male age or size.
The study also found that preferences based on visual traits differed significantly from those based on non-visual traits in out-of-season mares.
The stallions found to be generally preferred displayed a high libido.
Claus Wedekind, Sabine Meinecke-Tillmann and their colleagues, writing in the journal PLOS ONE, concluded that the oestrous cycle in mares had a significant influence on female preferences for visual and non-visual male traits.
The seven-strong study team said female preferences for male characteristics are expected to be driven by different selective forces.
Female animals therefore often base their choice on various traits that may signal different male characteristics, for example, odours, vocal signals, behavioural and visual displays, size, and secondary sexual traits such as antler size in some deer species.
The researchers set out to investigate possible links between female preferences and oestrous cycle in the horse.
Under feral conditions, horses either live in bands of breeding animals or of non-breeding stallions.
Ovulatory oestrous cycles typically occur in mares in spring and summer. The oestrous cycle lasts about 22 days, with 5–7 days in season with each cycle. Ovulation occurs about 24 hours before the end of being in season.
However, the timing and frequency of oestrous cycles also depend on the presence or absence of a stallion.
When in season, mares show sexual interest in stallions. When out of season, they can be aggressive towards them and seem to be generally less interested in interacting.
The researchers undertook an experiment involving 19 healthy non-pregnant mares during one breeding season. They were of different breeds.
The oestrous behaviour of the mares was monitored by teasing with a stallion that was not used in the experiment.
Seven unrelated stallions were used for the experiments, all from the Franches-Montagnes breed, with proven fertility and aged 5 to 19.
Experiments were performed using specially designed stables, with four stalls on either side of a corridor. The stables had 1.4m walls with a metal grille above.
For each test, the seven stallions were randomly assigned to the eight stables, and each mare was released into the corridor. The mare’s actions were recorded on cameras for analysis.
They aimed to test the females’ preferences four times during two oestrous cycles, twice when in season and twice when out of season.
In some situations the mares had vocal contact and could smell the stallion, and could even engage in a little physical contact, but had extremely limited visual contact through only a tiny opening.
In other situations, there was much better visual contact between them due to the removal of blinds.
The stallions were scored on a scale from 1 to 7 with regard to the choice of each mare, with a preference score of 1 for the stallion that received the highest contact time in the first test round, and a score of 7 for the stallion with least.
Bonding-type behaviours monitored included stance, hind legs posturing, tail raising, clitoris winking, urinating, and searching for stallion contact with the nose or pelvic area. Threatening behaviors such as rearing, kicking and biting were also noted.
“We found that female preferences for visual or non-visual male traits depend on the stage of their oestrous cycle,” they reported.
Visual traits were important when mares were out-of-season, but seemed to play a minor role when in season.
When not in season, mares showed a strong preference for older and larger stallions based on visual traits alone. When visual contact was prevented and only vocal or smell-related contact was allowed, female discrimination of stallions seemed no longer based on size or age.
In contrast, females in season showed preferences mainly based on non-visual traits that were not significantly influenced by male age and size.
“The relative importance of olfactory [smell-related] versus vocal traits remains unsolved here,” they said.
“Vocal displays have been shown to affect female hormone levels and to influence ovulation in several mammals.
“In our analyses, the vocal influences on the mares’ decisions could not be analysed and would have to be studied in a different experimental design.”
Female preferences have often been found to be influenced by olfactory displays in mammals. “The scent of a male may reflect his condition and how much it suffers from infections, allowing females to avoid infection or potentially get good genes.
“In our study, however, an effect of pathogens seems unlikely as all males were dewormed prior to the experiments and showed no signs of infections.”
Odours, they said, also revealed information about various important genes.
“Our observations suggest that male age, size, and probably other visual traits play no decisive role in the mating preference of oestrous mares.
“It remains to be elucidated in further studies what additional male characteristics might influence mares’ preferences in oestrus.”
The authors said they did not assess dominance ranking in their study, saying it should be assessed in further research.
“Some theories of sexual selection predict little variation in female preferences.
“Life history theory predicts female preferences to be often context dependent. We would therefore predict variance in female preferences when tested under different conditions.
“Here we found that female preferences in horses depend both on the mares’ oestrous cycle and whether or not visual traits are available.”
The full study team comprised Dominik Burger, Charles Meuwly, Selina Thomas, Harald Sieme, Michael Oberthür, Claus Wedekind and Sabine Meinecke-Tillmann. They are variously affiliated with the University of Berne, the University of Veterinary Medicine Hannover, and the University of Lausanne.
Burger D, Meuwly C, Thomas S, Sieme H, Oberthür M, Wedekind C, et al. (2018) Cycle-specific female preferences for visual and non-visual cues in the horse (Equus caballus). PLoS ONE 13(2): e0191845. https://doi.org/10.1371/journal.pone.0191845